MULTIDIMENSIONAL PROFILING USING SIMS 1 Why SIMS cannot be applied directly

Detection limits o f 1016 atoms cm 3 and below are regularly obtained for

SIMS depth profiles in semiconductor materials and in imaging experiments

lateral resolutions o f 3 0 nm have been reported. If both of these results could

be achieved simultaneously it would appear that direct mapping of the dopant distribution in two or even three dimensions should be possible. However, to obtain high lateral resolution a gallium liquid metal ion gun is used, this has little ion yield enhancement resulting in a poor detection limit. To gain the high yields produced in depth profiling a reactive bombarding species is required, however, to date, finely focussed reactive probes are unavailable.

It may be argued that this constraint can be overcome by loading the specimen surface with oxygen by thermal oxidation, or supplying a rich oxygen flood during analysis, and analysing with a liquid metal ion gun. Both of these would increase the ion yield and therefore lower the detection limit. However, the former would cause distortion of the profile and the latter would only partially increase the ion yield. This is because to obtain a fine probe high energies are required, hence the probe is highly penetrating and material from below the surface oxide (in an unoxidised state) may be ejected.

Even if a finely focussed reactive beam were available, or some other scheme to increase ion yields were employed, direct mapping still presents a

problem. Suppose a spatial resolution of 30 nm is desired for determination of

the dopant distribution of boron in silicon. Each voxel may therefore be

represented by a cube of side 30 nm. The atomic density of silicon is

very high useful yield of 1% is assumed then * 14 000 ions reach the detector. For a very modest statistical precision of 33% ten impurity ions must be

detected. The voxel must therefore contain at least 1000 impurity atoms,

representing an atomic concentration of 3.7xl019 atoms cm-3. Thus for a

particularly favourable combination of instrument and sample the detection limit is still two orders of magnitude worse than that required for verification of process modelling software. Figure 4.1 shows the variation of sensitivity with resolution for the described conditions.

Figure 44 Variation of

The experiment may be modified, to increase the amount o f material available, by sacrificing the third dimension and profiling only in two, in a similar way to that shown in figure 4.2. However, to recover information down

to 1017 atoms cm-3 the extension factor must be at least 100. Thus each voxel is

now 30 x 30 x 3000 nm. To maintain optimum spatial resolution (30 nm) the long direction of the analysed voxel must lie exactly parallel to the implanted feature. As this is not practical, some margin of error should be permitted. A

n

t o 100

sp a tia l res o lu tio n / nm 1000

30 nm error in the alignment would degrade the lateral resolution by a factor of two (to 60 nm), as adjacent regions are being sampled in one scan, and still require an alignment accuracy of 0.5°.

Figure 4 ,; py sacrificing <>ng dimension vq.xeI ya!umy may he extended to to^er thy detection limit

Added to the problem of alignment is the mixing effect o f the probe, as the LMIG must run at >30 keV to provide the high resolution. This effect will distort the profile as the analysis proceeds as, unlike static imaging, the underlying layers will be extensively mixed with the emissive surface.

The above examples are highly optimistic and results obtained using currently available instrumentation are likely to be worse by at least an order of magnitude (using reactive species and stigmatic imaging, limiting the spatial resolution to 500 nm, Bryan et. al. (2.3.2) achieved a sensitivity of only

1 0IS atoms cm-3).

Direct application of SIMS cannot at present provide the sensitivity with adequate resolution required for useful determination of dopant distribution in more than one dimension.

4.2.2 Indirect use of SIMS

In section 4.2.1 it was demonstrated that SIMS could not be applied directly to produce two (or more) dimensional dopant maps because there is simply not enough impurity material in each sampled element to provide

reasonable statistical precision. In depth profiling applications, where a lot

more material can be consumed, detection limits of the order of one part in 1 0*

are often achieved. This is much better than can be achieved with other

techniques possessing spatial resolution, eg Auger, and thus SIMS is already part way to providing the solution. To allow this potential to be realised in

multidimensional profiling the analyte volume must be increased independently

of the spatial resolution. This may be done by analysing many similar volumes simultaneously, although accurate registration between them must be maintained.

The ideal approach to solving this problem would be a specimen that may be analysed using existing instrumentation, that does not require extremely accurate registration o f microscopic features (ie not on the nm scale) and that can be analysed with reactive probes to enhance the ion yield.